(1) Field of Invention
The present invention pertains to an inkjet image forming apparatus employing ink droplets jetted from one or more printheads to form one or more images on one or more recording media and to an inkjet image forming method executed in the context of such an inkjet image forming apparatus. In particular, the present invention relates to improved measures adoptable when image forming operations are interrupted.
(2) Conventional Art
Conventional image forming operations in inkjet image forming apparatuses (hereinafter “inkjet printers”) are such that one sheet among a plurality of sheets of paper housed in a supply tray is first delivered to a transport path, this paper being supplied by way of the transport path to an image forming unit. In addition, at this image forming unit, ink droplets are jetted onto the surface of the paper to form a prescribed image, and this paper is thereafter discharged to an outlet tray.
Furthermore, in serial-head-type inkjet printers, a carriage provided at the image forming unit carries ink cartridges, the aforementioned image forming operations being carried out as a result of ink which is supplied to the printhead from the ink cartridges as the carriage performs scanning operations.
With such inkjet printers, in the event that an ink cartridge runs out of ink or some other problem occurs during the course of image forming operations, such image forming operations would have to be interrupted. Known as one disclosure of processing operations which may be carried out in the event that image forming operations are interrupted in such fashion is Japanese Patent Application Publication Kokai No. S62-2777 (1987). This publication discloses interrupting image forming operations and forcibly discharging the recording paper without further processing in the event that some abnormal event occurs (e.g., if supplied electric current exceeds some allowed peak value for a fixed period of time, etc.) while image forming operations are underway. In other words, the art disclosed in this publication is such that the recording paper is forcibly discharged following interruption of image forming operations even where circumstances are such that it would have been possible to resume image forming operations, continuing from where they had been left off, without ill effect. For this reason, not only does the recording paper go to waste, but the image forming operations which were being carried out prior to the interruption also go to waste.
Known as a solution to this problem is the inkjet image forming apparatus disclosed at Japanese Patent Application Publication Kokai No. H10-329333 (1998). This publication discloses interrupting image forming operations and resuming image forming operations without forcibly discharging recording paper following refilling of ink (cartridge replacement) when an ink cartridge runs out of ink while image forming operations are underway.
However, in recent years there is a trend toward increased number of nozzles on printheads in response to demand for increased speed of image forming operations. For example, in the aforementioned serial-head-type inkjet printer, by increasing the number of nozzles in the cross-scan direction of the printhead (the paper transport direction) it is possible to increase the size of the region over which image forming operations are carried out during a single scan pass, permitting the number of scan passes necessary to carry out image formation on a single sheet of recording paper to be decreased and permitting the time required for image formation to be reduced.
When thus increasing number of nozzles in the cross-scan direction of the printhead, there is a tendency for the dimension of the printhead in the cross-scan direction to increase in length. In accompaniment thereto, as shown at FIG. 10(a), distance T between shafts of pairs of rollers 42, 61 located to either side of printhead 52 in the cross-scan direction therefrom which respectively form nips for transport of recording paper P must also increase in length. The fact that this distance T between shafts of rollers increases means that the distance in the transport direction between the nips which sandwich and support the recording paper P during transport thereof will also increase in length.
Now, it is ordinarily the case that because the foregoing inkjet printer is such that image forming operations are carried out with ink droplets being jetted onto one side (the top) of recording paper P, the side of recording paper P onto which the ink droplets fall will swell in accompaniment to absorption of the ink. In accompaniment to this swelling at one side thereof, recording paper P will become bowed to some extent.
During normal image forming operations, because image formation on a single sheet of recording paper P is completed within a comparatively short time, this bowing of recording paper P is small and is rarely a problem.
However, in the event of interruption of image forming operations as a result of occurrence of some abnormal event during the course of image forming operations as described above, the foregoing swelling will progress and the bowing of recording paper P will increase as the time until resumption of image forming operations following such interruption grows longer. In particular, where the distance between shafts of respective rollers 42, 61 is large in order to accommodate a large number of nozzles at printhead 52 as described above, because the region of occurrence (area) of bowing on recording paper P will be large, there is a tendency for the amount of bowing (deflection) of recording paper P to increase. Under such conditions, as shown in FIG. 10(b), the surface of recording paper P may contact printhead 52. Where image forming operations are resumed without forcible discharge of recording paper P when operations are resumed, such as is disclosed at the aforementioned Japanese Patent Application Publication Kokai No. H10-329333 (1998), this can result in continuation of image forming operations on a sheet of recording paper P whose surface has become smeared or otherwise dirtied due to contact with printhead 52, and may in some cases even lead to paper jams and/or damage to printhead 52.
One approach for avoiding such contact of recording paper P with printhead 52 might be to set the distance between recording paper P and printhead 52 (dimension t at FIG. 10(a)) in advance such that it is a large value. But this is not preferred, since it would increase the flight distance traveled by ink droplets jetted from the nozzles of printhead 52 in reaching the surface of the recording paper, causing the location at which ink lands on the recording paper to shift and creating opportunities for deterioration in image quality.
Alternatively, when image forming operations are interrupted, it is possible that contact of recording paper P with printhead 52 might be avoided if printhead 52 were moved to a location (home position, etc.) at which it was not facing recording paper P. However, during resumption of image forming operations under such circumstances, if occurrence of the foregoing bowing were to cause recording paper P to reach the region through which printhead 52 passes (if recording paper P were to bow as far as the location shown in FIG. 10 (b)), the side of printhead 52 might collide with the edge of recording paper P, leading to damage of recording paper P and/or printhead 52.